Why would my guy friend turn silent and seem uninterested when I tell him about my ex?

So my guy friend that I haven't seen in a long time turned really quiet and kinda looked away when I explained things about my ex fiance. I could tell he was still listening but it seemed like he was bothered/felt uncomfortable or uninterested. Why would he do this? Mind you him and I go back many years. He even gave me a diamond bracelet when we were 11/12 years old, hung out as teens, lost touch only to be in contact again 10-15 years later.

Why would my guy friend turn silent and seem uninterested when I tell him about my ex?
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